The exemplary embodiment relates to systems and methods for limiting or detecting unauthorized copying of documents and finds particular application in connection with a system and method for encoding information in an original document using a single colorant, which is detectable under ultraviolet light in a copy of the original document but not in the original document.
As the quality of color copiers has improved, it has become easier to generate copies of a document which are indistinguishable from the original document. In many instances, the unauthorized copying of document content can have serious implications. For example, there is a concern that color copiers could be used to reproduce security documents, such as checks, stock certificates, automobile title instruments, and other documents of value, for illegal purposes.
One method which is used to authenticate documents and to reduce the unauthorized copying employs what is commonly called the VOID-pantograph. Techniques for creating such pantographs typically involve forming printed dots (or other elements) of two different sizes and frequencies, which are used to create regions of similar tone, corresponding to a textual warning and a background, respectively, in an original (authentic) document. Tone refers to the visual appearance produced by halftone dots, bars, or marks which cover at least a portion of a printed area and which usually have a frequency that is measured in dots, lines, or marks per inch. To provide constant tone, the smaller elements have a higher frequency than the larger elements. Because the tone of the textual warning and the tone of the background pattern are selected to be the substantially the same, these two regions have a similar visual impact on an observer of the original document, and the textual warning is not readily perceived.
On copying, however, the situation changes. Since the response of an image sensor employed in the scanner is different from the response of the human visual system, changes in the relative tone of the two different areas will appear. These changes are due to the different frequency response of the sensor (with respect to the human eye) and due to other, normally non-linear, effects, such as a detection floor or threshold, where signals below a certain level are simply “lost.” In general, the high frequency, smaller components are more strongly affected and attenuated. The difference in response of the scanner expresses itself as a relative change in tone in the copy and thus the hitherto invisible textual warning becomes visible. For example, in the resulting copy, only the larger printed dots are apparent. These larger dots spell out the word “VOID,” or other pre-determined textual warning.
The pantograph may be applied to the substrate to create a pre-printed carrier. An image to be protected is then applied to the pre-printed carrier. In other cases, the pantograph may be incorporated into the printed image.
A problem with the standard void pantograph is that it creates a readable message which a forger or counterfeiter can see when the original document is copied. The forger may then seek a different method for creating unauthorized copies.
Recently, fluorescent void pantographs have been developed which become visible only under UV light in the copy of an original document. To create the fluorescent void pantographs, patterns are printed with light and dark colors. These have an advantage over standard pantographs in that they are not visible (without magnification) under normal illumination, above the UV range of the spectrum. Such methods are not suitable, however, for monochrome printing devices.
The present system and method provide a method and apparatus for creating monochrome patterns for encoding information in documents and for detecting unauthorized copying. As an example, a forger or counterfeiter who tries to have a forged prescription filled can be caught at the pharmacy.